The present invention relates as indicated to an apparatus for retaining the attitude of a platform on an articulated boom, and relates more particularly to an apparatus which maintains the platform at a predetermined attitude relative to a fixed support while the articulating boom is raised or lowered. The apparatus of the present invention retains the predetermined platform attitude without the need for any operator intervention during the raising and lowering of the articulating boom. Additionally, the apparatus permits easy readjustment of the predetermined platform attitude.
Elevatable platforms for elevating workers to various work sites such as telephone lines or fruit-bearing trees, are commonly used. These platforms are typically elevated by a hydraulically operated boom or crane assembly. The boom assembly generally includes a support boom and an articulating boom hinged to the support boom whereby the articulating boom can be pivoted relative to the support boom so as to raise or lower the attached platform.
Undesirable consequences can occur if the platform is fixed relative to the main boom since the attitude of the platform will change correspondingly as the articulating boom changes its orientation relative to the support boom. In the case where the platform is occupied by a worker, injury could result if the worker cannot maintain his footing on the tilting platform bottom. Even if the platform is not occupied by a worker, injury to those on the ground can occur if materials or heavy objects are spilled or thrown from the tilting platform.
A number of solutions to the problem referred to above have been proposed. Judged against the important criteria of cost, reliability and ease of operation, however, the existing solutions have still left room for improvement. Additionally, none of the existing solutions have satisfactorily addressed the problem of maintaining the platform attitude while both the support boom and the articulating boom are independently moved. While apparatuses have been proposed in which the attitude of the platform can be adjusted relative to either the support boom or the articulating boom, there has heretofore not existed a reliable, practical apparatus which adjusts the platform attitude while both booms are simultaneously moving.
An analysis of several existing apparatuses reveals the deficiencies of these apparatuses in maintaining platform attitude when both the support boom and the articulating boom are moving. U.S. Pat. No. 3,828,939 to Tranchero illustrates a crane for raising and lowering a work platform and using a parallelogram arrangement to retain the horizontal attitude of the bottom plane of the platform. Although the apparatus disclosed in Tranchero maintains the platform at a horizontal orientation without any operator intervention, the apparatus has several disadvantages. For example, no mechanism for changing the orientation of the platform from a horizontal orientation to another orientation is provided. Also, the lever parallelogram arrangement precludes the use of a boom which can be telescoped to increase its length.
Several devices using a master-slave cylinder arrangement have also been proposed to maintain the orientation of an aerial platform. These devices include a slave cylinder operatively connected to the platform which extends or retracts in correspondence with the extension or retraction of a master cylinder. For example, U.S. Pat. No. 3,866,713 to Carpenter et al discloses an apparatus for extending an operator platform having a master cylinder connected between the articulating boom and the support boom. A slave cylinder is connected at one end to the fly end of the articulating boom and connected at its other end to the platform. The master and slave cylinders are interconnected in a closed circuit such that extension and retraction of the master cylinder effects corresponding retraction and extension of the slave cylinder. However, Carpenter does not disclose an arrangement for maintaining the platform orientation while both the support boom and the articulating boom move independently. Separate master cylinders would be required to track the independent movements of each boom and an appropriate slave cylinder arrangement would be required to produce corresponding movement of the platform in response to the movement of each master cylinder. Applicants are unaware of arrangements in which multiple master cylinders are connected to a single slave cylinder. Presumably this is because the relationship of cylinder length to boom angle is a cosine function rather than being linear. As a result, with independently operated booms, the platform leveling slave cylinder is easily unsynchronized.